1. Field of the Invention
This invention relates to devices and methods for using a magnetic field to guide a surgical implant, and more specifically to devices and methods for using the near field and transition field of a repositionable magnet to move, guide, and/or steer a magnetic seed, catheter or other magnetic delivery vehicle (MDV) for therapeutic or surgical purposes.
2. Description of Related Art
In the field of surgery, there exists a need to control the orientation, forces, and/or motion of internally implanted devices. One method that has been used to control such implanted devices is the application of a magnetic field from an external magnet. In this method, the magnetic field acts upon the implanted device, which itself comprises magnetic material, which may be in the form of a permanent magnet. In accordance with prior art practice, a physician surgically implants the device comprising magnetic material and then guides the position of the implanted device by moving an external permanent magnet and observing the resultant movement directly with an X-ray fluoroscope. Examples of the prior art may be found in a review article by Gillies et al., xe2x80x9cMagnetic Manipulation Instrumentation for Medical Physics Research,xe2x80x9d Rev. Sci. Instrum. 65, 533 (1994), and references cited therein. See also McNeil et al., xe2x80x9cFunctional Design Features and Initial Performance Characteristics of a Magnetic-Implant Guidance System for Stereotactic Neurosurgery,xe2x80x9d IEEE Trans. Biomed Engrg., 42, 793 (1995); Tillander, xe2x80x9cMagnetic Guidance of a Catheter with Articulated Steel Tip,xe2x80x9d Acta Radiologa 35. 62 (1951); Frei et al, xe2x80x9cThe POD (Para-Operational Device) and its Applications,xe2x80x9d Med. Res. Eng. 5,11 (1966); U.S. Pat. No. 3,358,676 to Frei et al., issued Dec. 19, 1967, entitled xe2x80x9cMagnetic Propulsion of Diagnostic or Therapeutic Elements Through the Body Ducts of Animal or Human Patientsxe2x80x9d; Hilal et al., xe2x80x9cMagnetically Guided Devices for Vascular Exploration and Treatment,xe2x80x9d Radiology 113, 529 (1974); Yodh, et al., xe2x80x9cA New Magnet System for Intravascular Navigation,xe2x80x9d Med. and Biol. Engrg., 6, 143 (1968); Montgomery et al., xe2x80x9cSuperconducting Magnet System for Intravascular Navigation,xe2x80x9d Jour. Appl. Phys. 40, 2129 (1969); U.S. Pat. No. 3,674,014 to Tillander, issued Jul. 4, 1972, entitled xe2x80x9cMagnetically Guidable Catheter-Tip and Methodxe2x80x9d; and U.S. Pat. No. 3,794,041 to Frei et al., issued Feb. 26, 1974, entitled xe2x80x9cGastrointestinal Catheter.xe2x80x9d The full content of each of the cited documents are herein incorporated by reference in their entirety.
Unfortunately, the above-described technique requires the physician to react to the movement of the implanted device after the fact. There is no precise correlation of the imaging system with the medical magnetic manipulation, and no way to apply fields and/or force gradients precisely in needed directions. With hand-held magnets, the only feedback the surgeon could have was his observation of motion of a magnetic implant by x-ray or ultrasonic imaging in response to his movement of the magnet. The field producing magnet, so guided without direct visual display of the field lines, is controlled by the operator""s estimate of the field direction and magnitude at a particular location of the implant. Since many combinations are possible, this essentially xe2x80x9cblind operationxe2x80x9d is bound to result in a somewhat random position and angulation as related to the needed field line direction and magnitude to provide guidance and/or pulling force. In difficult interference situations, it is difficult without such imaging guidance to provide even a reasonable guess as to a correct direction for the magnet axis to obtain field alignment with the intended path. The large electromagnet of Yodh et al. is one attempt to minimize the xe2x80x9cblindnessxe2x80x9d of the approach just described, but the Yodh et al. approach still relies on operator judgment and vision, and is subject to such error. While multiple coil arrangements such as the magnetic stereotaxis system (MSS) described in McNeil et al. can be used to provide such guidance, it is difficult in such systems to provide a combined guiding force and force-applying field gradient in the same desired direction.
Clearly, both operation time and risk to a patient could be reduced if an apparatus and method were available to more accurately and rapidly guide or move a magnetic surgical implant. This device and method can either provide feedback to the physician guiding the implant so that the physician could predict the movement of the implanted device rather than react to it after the fact, or it can be used more automatically with computer-controlled motion along a physician-selected planned path. It would also be advantageous if simpler hardware and software could be used to locate the external magnet and provide more effective field solutions. The moveable magnet location should take into account an exclusion volume around the patient in which the magnet may not be located. In the case of neurosurgery, for example, the magnet cannot be located too closely to the patient""s head, nor in the path of imaging X-rays.
It is thus an object of the invention to provide a rapid interactive display of the aligning torque and magnetic pulling directions of a magnet acting on a volume.
It is a further object of the invention to provide a physician with devices and methods that facilitate the prediction of the movement of a magnet implant in response to an externally applied magnetic field.
It is yet another object of the invention to provide devices and methods that facilitate rapid and appropriate adjustment of the position of an external magnet to steer a magnet internal to the body of a patient.
It is still another object of the invention to provide a moveable field-producing magnet that can be located and angled so as to provide flexibility in avoiding interference with imaging systems which may change between and during various surgical procedures.
It is another object of the invention to provide means whereby a physician can use voice control or other non-tactile control to govern path choice at arterial branches or lumenal branches so that both hands are available for other needs, in intravascular navigation applications.
It is yet a further object of the invention to provide an external magnet adapted to provide a magnetic field of sufficient strength and appropriate angular spread to provide flexibility in positioning and orienting the external magnet, even while respecting exclusion volumes around a patient""s body.
These and other objects are achieved by the inventive methods and apparatuses to guide an implant disclosed herein. The invention provides rapid interactive display of the aligning torque and magnetic pulling directions of a permanent, a superconducting, or a resistive wire magnet acting on a volume, which volume may include a portion of a patient""s body. The invention allows a hand-held, hand-positionable or servo-controlled external magnet to be moved external to the volume, while the resultant magnetic forces are displayed in real-time (essentially instantaneously, or at least as rapidly as is needed for effective surgical control) on a computer screen. In this way, a physician can rapidly adjust the position of an external magnet to steer a magnet internal to the body.
By placing a set of fixed fiducial marks on the magnet and using a device that can localize these marks in three dimensions, the position of the magnet can be associated with the treatment volume. The volume""s position can be associated with the magnet by placing fiducial marks on the volume and xe2x80x9cregisteringxe2x80x9d these marks with the localizer, or by putting the volume in a standard place relative to the localizer. A second method of localizing the magnet includes putting magnetic sensors at appropriate fiducial points.
The magnetic field of any magnet can be simply measured and suitably mapped in three dimensions. At run time a pre-measured map can be superimposed on the imaging volume using the registration established by the fiducial marks.
Assuming that the volume has a set of images associated with it and fiducial marks that can be registered in the imaging volume, the magnetic field and force can be displayed and updated as the magnet is moved relative to the imaging volume. The field can be displayed at any point in the image set, and updated as the magnet is moved. Commercial ultrasonic and infrared localizers exist at present that give great accuracy (on the order of a millimeter) to this technique. One such localizer is described in U.S. Pat. No. 5,383,454, issued Jan. 24, 1995 to Richard D. Bucholz, which is hereby incorporated in its entirety by reference.
In one embodiment, an apparatus in accordance with the invention comprises a moveable magnet assembly having a plurality of fiducial marks thereon; a localizer comprising a plurality of imaging sensors configured to sense a position in three-dimensional space of the fiducial marks to thereby provide an indication of a location of the moveable magnet; and a processor including memory having stored therein a pre-measured representation of a magnetic field generated by the magnet assembly and a display configured to present a graphical representation of the magnetic field, the processor being configured to be responsive to the indication of the location of the moveable magnet provided by the localizer so that the display provides an indication of the magnetic field within a selected volume of space resulting from the magnet assembly when the magnet is at a position indicated by the localizer.
In accordance with another embodiment of the invention, a method for providing a rapid interactive display of the aligning torque and magnetic pulling directions of a magnet comprises the steps of providing a plurality of fiducial marks on the magnet assembly; sensing a location and orientation of the fiducial marks; computing a magnetic field in a selected volume of space produced by the magnet assembly when the magnetic assembly is in the sensed location and orientation; and displaying a representation of the magnetic field in the selected volume of space. This inventive method embodiment may further comprise the steps of placing a magnetic seed in a tissue of a living body; providing fiducial marks on an external surface of the living body proximate the tissue; sensing a location and orientation of the living body; and selecting the selected volume of space to include the tissue of the living body.
In another variation of the invention, the field lines need not be displayed. Instead, a vascular or other lumenal path displayed on the screen is surgeon identified as being the path of choice. The computer is then able to xe2x80x9cknowxe2x80x9d in the treatment volume the direction needed for the guiding field at each point along the path. It can then provide orientation and positioning of the external, field-generating magnet as needed for navigation along the path. At arterial or other lumenal branch points, the surgeon can further assist the delicate navigational needs by voice commands understandable by the computer.
In other variations of the inventive methods and apparatuses, means are provided for locating and angling the external field producing magnet while avoiding interference with the imaging means, which may include an exclusion volume about a patient""s body. In yet another variation, a permanent, superconducting, or resistive wire external field-generating magnet provides a xe2x80x9cshapedxe2x80x9d field, i.e., the near (and/or transition) field of the magnet is produced by a specially shaped coil and/or core to provide magnetic fields of sufficient strength and appropriate angular spread to provide additional flexibility in positioning the magnet during surgery. In the case of a superconducting magnet, the winding shape is adjusted. For a resistive wire magnet, the windings and core of the magnet may both be adjusted to accomplish the shaping.
These and other embodiments of the invention may be fully understood by those skilled in the art by reference to the drawings and to the detailed explanation that follows.